Evaluation of poly(styrene-d5) and poly(4-fluorostyrene) as internal standards for microplastics quantification by thermoanalytical methods

Thermoanalytical methods such as pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) are among the most promising techniques for the quantification of microplastics (MP) in environmental samples. However, methods still lack harmonization and standardization. The use of an internal standard (IS) can improve the quality of quantitative data and the robustness of the analytical method. Poly(styrene-d5) has frequently been used as IS for MP quantification, but suffers from H–D exchange during pyrolysis, which is catalyzed by inorganic components of the sample matrix, most evidently in aluminum oxide filter matrix and in sea sand. Poly(4-fluorostyrene) (PFS) is a promising alternative, which does not suffer from exchange reactions. Both standards yielded comparable results in calibrations for polypropylene, polyethylene, and polystyrene MP, but a larger concentration range could be covered if PFS was used. Only minor interferences of both polymers with natural organic matrix components were observed. The study of influences of various inorganic matrices on pyrolysis of the polymers revealed manifold effects that may interfere with MP quantification and underline the need for matrix-matched calibration or extensive sample clean-up.